Method of preparing analyte material for spectrochemical analysis

ABSTRACT

An analyte material is prepared for spectrochemical analysis by depositing it in solution or electrostatically precipitating it upon a previously purged yarn of finely divided carbon fibers, dessicating it in situ, and electrically heating the yarn sufficiently to vaporize the analyte material while passing a gas stream over it in which the analyte material is condensed in particles of sufficiently small size to form an aerosol which is then conducted to any of a variety of excitation sources for spectrochemical analysis.

United States Patent 1191 1111 3,832,060 Dahlquist Aug. 27, 1974 METHODOF PREPARING ANALYTE 3,419,359 12/1968 Anderson et 356/87 ux MATERIALFOR SPECTROCHENHCAL 3,693,323 9/1972 Gant 1. 55/74 X ANALYSIS [75]Inventor: Ralph L. Dahlquist, Santa Barbara,

Calif.

[73] Assignee: Applied Research Laboratories, lnc.,

Sunland, Calif.

[22] Filed: Aug. 10, 1972 [21] Appl. No: 279,619

[52] U.S. Cl 356/36, 356/74, 356/85 [51] Int. Cl. G0ln l/00 [58] Fieldof Search 356/36, 38, 8587,

[56] References Cited UNITED STATES PATENTS 3,174,393 3/1965 Dewey etal. 356/187 Primary Examiner-John K. Corbin Assistant ExaminerF. L.Evans Attorney, Agent, or Firm-Theodore H. Lassagne ABSIRACT An analytematerial is prepared for spectrochemical analysis by depositing it insolution or electrostatically precipitating it upon a previously purgedyarn of finely divided carbon fibers, dessicating it in situ, andelectrically heating the yarn sufficiently to vaporize the analytematerial while passing a gas stream over it in which the analytematerial is condensed in particles of sufficiently small size to form anaerosol which is then conducted to any of a variety of excitationsources for spectrochemical analysis.

12 Claims, 1 Drawing Figure METHOD OF PREPARING ANALYTE MATERIAL FORSPECTROCHEMICAL ANALYSIS BACKGROUND OF THE INVENTION 1. Field of theInvention The present invention relates to a method of preparing analytematerials for spectrochemical analysis and more particularly to such amethod which is compatible with the requirements of various conventionalanalytical modes such as, for example, atomic emission, atomicabsorption and atomic fluorescence.

2. Description of the Prior Art Spectrochemical analysis is among themost sensitive of all analytical methods, a few milligrams of a sampleusually sufficing for the detection of elements present to the extent ofonly parts per million. For this reason it is widely used in analyticaland clinical chemistry where extremely small sample volumes areavailable.

In spectrochemical analysis, atoms of an analyte; i.e., a sample of thematerial to be analyzed, are excited to energy levels above theirnormal, or ground, states and caused to emit or absorb light ofcharacteristic frequencies as they return to lower energy states. Thesecharacteristic frequencies are separated into an ordered sequence, orspectrum, by diffraction or refraction for observation and/or recording.Each chemical element has a unique, characteristic collection ofspectral frequencies whose presence, or absence, in the spectrum of anunknown substance presented for analysis provides unambiguous evidenceof the presence of that element.

The preparation of materials for spectrochemical analysis hasconventionally been effected by nebulization of solutions of suchmaterials into liquid droplets which are then introduced into a flame orother excitation medium. However, analytical results obtained by suchnebulization are subject to many effects deleteriously affectingaccuracy of the result. Viscosity, volatility and surface tension affectnebulizer efficiency and hence analytical results.

Alternatively, analyte material has been coated directly on carbon ormetal tubes or filaments which have then been heated, usuallyelectrically, to effect atomization and excitation of the atoms of thematerial in a single step. This procedure, however, has requiredpreparation of the tube or filament by preheating for extended periodsto diffuse to the surface and evaporate impurities and, desirably,metallic coating of the tube or filament, if of carbon, has beenrequired to prevent too rapid a diffusion of atomic vapor into thecarbon. Such devices all serve as atom reservoirs and are usedprincipally in absorption or fluorescence spectrocopy, althoughmicrowave plasmas are sometimes employed as emission sources where thesample is evaporated from a metal filament.

Dewey et al. in U.S. Pat. No. 3,l74,393 of Mar. 23, 1965 describe theemployment of a cloth of fine carbon fibers as a vehicle by means ofwhich analyte material deposited thereon in either aerosol or liquidform may be exposed to a flame or are, or heated by a current passedthrough the cloth itself, to effect excitation of atoms of the analytefor emission spectroscopy, while West et al., in Atomic'Absorptiori andFluorescence Spectropscopy with A Carbon Filament Atom Reservoir"published in Analytica Chimica Acta, 45 (1969) 27-41, describe variousconfigurations of apparatus for the excitation of analyte materials inatomic absorption and atomic fluorescence spectroscopy.

It' is a primary object of the present invention, however, to provide amethod and apparatus for the preparation of analyte materials forspectrochemical analysis which is independent of the procedure employedin the analysis itself and therefore is equally useful in atomicemission, atomic absorption and atomic fluorescence spectroscopy.

SUMMARY OF THE INVENTION According to the present invention, an analyteis divided into minute separate particles; i.e., particulated, withoutnecessarily being atomized in the sense of dissociated into free,single, neutral atoms, and without being nebulized in the sense of beingformed into liquid droplets.

So particulated, the analyte is conveyed by a stream of gas to anyselected excitation medium for spectral analysis. Such analysis can beeffected in any of three analytical modes; e.g., atomic absorption,atomic fluorescence, or atomic emission, because the sample preparationand its excitation are wholly independent of each other.

The particulation medium employed for this purpose is a yarn of finefibers of high purity graphitic carbon characterized by high thermal andgood electrical conductivity, adapting it for resistance heating bypassage of an electric current through it.

After a very brief pre-burn, analyte material is deposited directly onthe yarn which can accommodate relatively large quantities, of the orderof three microliters per centimeter of length, of liquid analyte due toits capillarity. Thus the analyte material is spread over an enormoussurface as compared with the area over which it may be spread in othernon-flame devices. This facilitates and expedites desolvation.

Desolvation is accomplished by heating the yarn to a temperaturesufficient only to evaporate the solvent and to remove minimal amountsof water of crystallization. If mercury or other volatile elements arebelieved present in the analyte, special care if of course required inthis step as with other techniques for analyte preparation.

Alternatively, in applications where it is desired to effectspectrochemical analysis of minute particles carried in the air or ingaseous discharges from industrial processes, a length of theelectrically conductive yarn may be employed asthe collector element inan electrostatic .precipitator, in which case the particles collectingon the individual fibers of the yarn will constitute the analytematerial.

Desolvation in the sense of evaporation of a solvent is not of coursenecessary in the preparation of analytes deposited on yarn in anelectrostatic precipitator, but dessication of the analyte to somepredetermined relative humidity can, if desired, be effected by heatingthe yarn carrying the electrostatically deposited analyte to atemperature sufficient only for that purpose.

Following the deposition of the analyte material on the fibers of theyarn in either of the ways described above, the yarn is heated to atemperature sufficiently which may be located at any convenient distancefrom the apparatus in which the particulation of the analyte iseffected. The temperature to which the yarn is heated for this purposeis sufficient to apparently vaporize the analyte material which is theninstantly condensed in the stream of carrier gas into particles ofsufficiently small size to form an aerosol.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a diagrammaticillustration of apparatus for preparation of an analyte according to themethod of the present invention; the arrows therein indicated directionsof gas flows.

DESCRIPTION OF THE PREFERRED EMBODIMENT According to the method of thepresent invention the material to be analyzed is deposited upon thesurfaces of the fibers composing a length of yarn of high purityvitreous graphitic carbon fibers a majority of which are, typically, ofdiameters in the range of four to eight microns.

Such vitreous graphitic fibers have been produced by processingcarbonaceous materials at temperatures up to 3,000 C. They are stable upto about 3,600 C. at which temperature they sublime without melting.Yarn formed of such fibers is of great strength and retains its strengthat elevated temperatures. It is electrically conductive and thereforecan be heated by induction or by the passage of an electrical currentthrough it. One such yarn is marketed by Union Carbide Corporation underthe trademark Thornel."

Preliminarily, this yarn is purged by heating it in an inert atmosphere,preferably by the passage of an electrical current through it, to atemperature sufficient only to diffuse to and evaporate from the surfaceof the fibers any impurities present; a stream of inert gas, such asnitrogen, being flowed over the surface of the yarn fibers during suchheating to carry away vaporized materials evolved. Because theindividual fibers composing the yarn are so small and finely divided,the yarn comes to firing temperature very quickly and the diffusion ofimpurities out of the material and their evaporation from the surfacethereof occurs in only a few seconds as contrasted with the many minutesrequired for impurities to diffuse to the surface and be evaporated frommore massive structures such as conventional non-flame cells. It hasbeen found, therefore, that with Thornel yarn, such as described above,a five second pre-burn period reduces impurities to non-detectablelevels; i.e., less than grams absolute. Also because of the vitreousnature of the yarn, provisions for the avoidance of soak-in anddiffusion are unnecessary.

Following such purging, material to be analyzed, i.e., the analyte, isdeposited on the surfaces of the fibers composing the yarn. Where theanalyte material is in liquid form, it will be observed that the yarncan accommodate relatively large quantities, of the order of threemicroliters per centimeter of length, of liquid analyte due to itscapillarity. Thus the analyte material is spread over an enormoussurface as compared with the area over which it may be spread inconventional non-flame devices.This facilitates and expedites drying andashing of analyte solutions without splatter or crusting losses.

Alternatively, in applications where it is desired to effectspectrochemical analysis of minute particles carried in the air or ingaseous discharges from industrial processes, such particles may bedeposited on the surfaces of the fibers composing the yarn by employinga length of the previously purged yarn as an electrically chargedcollector element in an electrostatic precipitator through which the airor gaseous discharge is passed. The particles of materials to beanalyzed are thus collected on the surfaces of the yarn fibers andseparated from the gaseous constituents of the atmosphere in which theywere carried.

Following the deposition of an analyte in liquid form, as describedabove, the analyte-impregnated yarn is again heated in an inertatmosphere, preferably by the passage of an electrical current throughthe yarn to a temperature sufficient to evaporate the solvent presentwithout dislodging the analyte material from the surfaces of the yarnfibers. During this step also, a stream of an inert gas, such asnitrogen, is flowed over the surface of the yarn fibers so as to carryaway vaporized materials evolved during such heating of the yarn.

This desolvation step is not necessary and may be omitted in connectionwith the preparation for analysis of materials electrostaticallydeposited on the yarn, although some dessication of electrostaticallydeposited analyte material may be effected to bring it to a desiredrelative humidity.

Finally, following desolvation or dessication, the yarn is heated onceagain to a temperature sufficient to vaporize analyte material from thesurfaces of the yarn fibers. During this heating, a stream of gas at atemperature at which the vaporized analyte material instantly condensesis flowed over it and conducted from that space to any of a variety ofexcitation sources at which the finely particulated analyte materialcarried away from the surfaces of the yarn fibers may be excited forspectrochemical analysis.

It will be evident from the foregoing that the present method ofpreparing analyte materials for spectrochemical analysis is entirelyseparate and independent of the analysis itself and that therefore it isuseful with any of a variety of analytical techniques such as, forexample, atomic absorption, atomic fluorescence or atomic emission.

Furthermore, the method of the present invention is adapted foremployment in the continuous analysis of materials which may be ineither liquid form or in the form of solid particles carried in theatmosphere or gaseous discharges from industrial processes, or the like.

As diagrammatically illustrated in the FIGURE, liquid analyte materialsmay be prepared for continuous spectrochemical analysis by continuouslyfeeding yarn 10 of the kind which has been described herein, from asupply reel 12 around a tension capstan 13' and over a pulley 14 into atube 15.

In a first portion 16 of the interior of the tube 15, a length of yarn10 is heated to a temperature of the order of 2,300 C. for from two tofive seconds by an electrical current passed through it under control ofa switch 17 from a suitable source via leads 18 to contacts such ascarbon rollers 19 contacting the yarn l0.

During such heating, an inert gas such as, for example, nitrogen, isflowed through the portion 16 of the tube 15; being introduced from anysuitable source via inlet 20 and discharged via outlet 21. Impuritiesevaporated by such heating from the surfaces of the fibers composing theyarn are carried away by this gas flow.

The continuously fed yarn then passes through the intermediate portion25 into another portion 26 of the interior of the tube 15. In thisportion 26 of the tube 15, a liquid analyte is continuously depositedupon the yarn during its movement by means diagrammatically illustratedat 27. During its movement through the por tion 26, the yarn 10 isheated to, typically, 80 C. by means such as a source of radiant heat28, such temperature being only sufficient to effect evaporation of anysolvent present.

During such heating, an inert gas, such as, for example, nitrogen, isflowed through the portion 26 of the tube being introduced from anysuitable source via inlets and 30 and discharged via outlet 32. Theevaporable constituents of the analyte solution are carried away by thisgas flow and the remaining analyte material is dried by the radiantheat.

The continuously fed yarn then passes through the intermediate portion34 into a third portion 36 of the interior of the tube 15. During itsmovement through this portion of the tube 18, the yarn is again heatedto a temperature of 2,000 c. by a high frequency electrical currentpassed through a coil 40 surrounding the portion 36 of the tube 15.

During such heating analyte material is vaporized and then recondensedinstantly in extremely finely divided particles which are carried awayfrom the surface of the fibers composing the yarn 10 by a stream of acarrier gas which may be either an inert gas such as, for example,nitrogen, or, where a flame is to be used for excitation of the analytein a concurrent spectrochemical analysis, a reducing gas combustible inthe presence of oxygen but inert with respect to the analyte materialitself in the absence of oxygen. This gas is supplied at essentiallyroom temperature through the inlet 30 and aspirated, together with theentrained analyte particles, to any desired excitation source throughoutlet 45, an excess discharging through outlet 46. The yarn 10 thenpasses through the outlet 46 and over a take-up capstan 47.

For the continuous preparation for analysis of materials in the form ofsolid particles carried in the atmosphere or in gaseous discharges fromindustrial processes and the like, yarn, after purging by means such ashas been described, is removed from the apparatus and employed as acollector element in an electrostatic precipitator until sufficientanalyte material has been collected on its fibers; the electricallyconductive yarn being electrically charged during such precipitation.Thereafter it is reintroduced into apparatus corresponding to the tubeportions 26 and 36 and associated elements for dessication of theanalyte to the desired relative humidity and its particulation.

In the foregoing description the means for heating the yarn in theportions 16 and 36 of the tube 15 are given as examples only, either ofwhich may be substituted for the other or for the heating meansdescribed for heating the yarn in the portion 26 of the tube 15 providedonly that in the latter case the electrical current is controlled toeffect heating only to the lower temperature required for desolvationand/or dessication.

What is claimed is:

1. A method of preparing analyte materials for spectrochemical analysiswhich comprises the steps of depositing analyte material in the solidstate on the sur' faces of the fibers of a yarn composed of fibers ofhigh purity graphitic carbon a majority of which fibers are typically ofdiameters in the range of four to eight microns, heating said yarn to atemperature sufficient to vaporize said solid analyte material,condensing said vaporized material into solid particles of sufficientlysmall size to form an aerosol and transporting such particles forspectrochemical analysis to a location optically remote from said yarnby passing a stream of gas over said yarn during said heating.

2. A method according to claim 1 in which the heating of said yarn iseffected by passing an electrical current therethrough.

3. A method according to claim 1 in which the heating of said yarn iseffected by induction.

4. A method according to claim 1 in which the steps described areperformed in a continuous sequence while moving said yarn between andthrough separate zones in each of which one of said steps is performed.

5. A method according to claim 1 in which the deposition of analytematerial on the surfaces of the fibers of said yarn is effected byimposing an electrical charge upon said yarn while exposing it to anatmosphere containing particles in the solid state of material to beanalyzed.

6. A method according to claim 5 in which the heating of said yarn iseffected by passing an electrical current therethrough.

7. A method according to claim 5 in which the heating of said yarn iseffected by induction.

8. A method according to claim 5 in which the steps described areperformed in a continuous sequence while moving said yarn between andthrough separate zones in each of which one of said steps is performed.

9. A method according to claim 1 in which said analyte material is firstdeposited on the surfaces of said fibers in solution in a solvent andwhich includes the additional step of heating said yarn onlysufficiently to evaporate said solvent prior to heating said yarn to ahigher temperature sufficient to evolve the analyte material therefrom.

10. A method according to claim 9 in which the heating of the yarn iseffected by passing an electrical current therethrough.

11. A method according to claim 9 in which the heating of the yarn iseffected by induction.

12. A method according to claim 9 in which the steps described areperformed in a continuous sequence while moving said yarn between andthrough separate zones in each of which one of said steps is performed.

1. A method of preparing analyte materials for spectrochemical analysiswhich comprises the steps of depositing analyte material in the solidstate on the surfaces of the fibers of a yarn composed of fibers of highpurity graphitic carbon a majority of which fibers are typically ofdiameters in the range of four to eight microns, heating said yarn to atemperature sufficient to vaporize said solid analyte material,condensing said vaporized material into solid particles of sufficientlysmall size to form an aerosol and transporting such particles forspectrochemical analysis to a location optically remote from said yarnby passing a stream of gas over said yarn during said heating.
 2. Amethod according to claim 1 in which the heating of said yarn iseffected by passing an electrical current therethrough.
 3. A methodaccording to claim 1 in which the heating of said yarn is effected byinduction.
 4. A method according to claim 1 in which the steps describedare performed in a continuous sequence while moving said yarn betweenand through separate zones in each of which one of said steps isperformed.
 5. A method according to claim 1 in which the deposition ofanalyte material on the surfaces of the fibers of said yarn is effectedby imposing an electrical charge upon said yarn while exposing it to anatmosphere containing particles in the solid state of material to beanalyzed.
 6. A method according to claim 5 in which the heating of saidyarn is effected by passing an electrical current therethrough.
 7. Amethod according to claim 5 in which the heating of said yarn iseffected by induction.
 8. A method according to claim 5 in which thesteps described are performed in a continuous sequence while moving saidyarn between and through separate zones in each of which one of saidsteps is performed.
 9. A method according to claim 1 in which saidanalyte material is first deposited on the surfaces of said fibers insolution in a solvent and which includes the additional step of heatingsaid yarn only sufficiently to evaporate said solvent prior to heatingsaid yarn to a higher temperature sufficient to Evolve the analytematerial therefrom.
 10. A method according to claim 9 in which theheating of the yarn is effected by passing an electrical currenttherethrough.
 11. A method according to claim 9 in which the heating ofthe yarn is effected by induction.
 12. A method according to claim 9 inwhich the steps described are performed in a continuous sequence whilemoving said yarn between and through separate zones in each of which oneof said steps is performed.